Relaying of wireless communications is a well-known tool to extend transmission ranges and to cover holes in coverage patterns. In surface-covering systems another known means to achieve radio coverage of, e.g., holes in coverage patterns is to install a transmission site, e.g., a base station at essentially the same position as the relaying equipment. When it comes to range extending, an underlying problem is often how to bridge a transmission range comprising obstacles in the environment, such as mountains or high buildings. In such situations, known alternatives to a relay station is to increase antenna mast heights or change position of at least one of transmitting and receiving stations to avoid obstacles interrupting or affecting the transmission path and possibly achieve a line of sight connection. However, mountains and buildings can reach considerable heights and their extensions can be substantial.
In radio communications systems for mobile communications it is not unusual that various base stations are interconnected by means of radio wave links, thereby eliminating interconnection of the base stations by wire.
International Patent Application WO2006121381 describes a method and arrangement for a wireless communications network using relaying. Distributed delay diversity is achieved by each relaying node cyclically shifting OFDM symbols a particular number of symbols in a sequence of symbols. In the international patent application both regenerative and non-regenerative relaying are considered.
International Patent Application WO2005064872 reveals a method and system for a wireless communications network using cooperative relaying. Base stations adapt transmissions to relay stations based on reported soft associations and channel quality measures. The international patent application includes embodiments making use of MIMO (Multiple Input Multiple Output) communications in the various wireless communications links.
FIG. 1 illustrates schematically a cellular system using relaying according to prior art. The figure shows one cell <<205>> of a wireless network comprising a transmitting and receiving node in the form of an access point or base station <<210>>. The network also comprises a number of relaying nodes <<215>>, <<221>>, <<222>>. Connections are established between the access point or base station <<210>> and served nodes or user equipment <<220>> over the relaying nodes as appropriate. The served nodes or user equipment are subsequently referred to as user equipment for short. The user equipment may be, e.g., a mobile station, a personal computer comprising wireless communications equipment. The relay-station antennas can be mounted on, e.g., antenna-masts or rooftops. The actual station relaying communications between the access point or base station <<210>> and a particular example mobile station <<220>> may change during the communications session, e.g., due to the user of the mobile station carrying the mobile station around, or reflecting/blocking objects affecting the radio paths are moving.
The acronym MIMO is used in prior art to refer to both channel properties and diversity or multiplexing properties of communications. As regards the channel properties, the communications channel which is achieved by using transmit and receive antennas with multiple sending and receiving antenna elements, respectively, is called a MIMO channel. As regards diversity or multiplexing properties, there are particularly two aspects worth mentioning:                1. Spatial multiplexing and        2. Diversity coding.        
In spatial multiplexing multiple lower rate data streams are, e.g., transmitted from different transmit antenna elements, normally in the same frequency channel. If these signals arrive at the receiver antenna with sufficiently different spatial signatures, the receiver can separate these streams, distinguishing the various data streams. The maximum number of spatial streams is limited by the lesser in the number of sending/receiving antenna elements at the transmitter or receiver.
Diversity coding is used, e.g., when the transmitter has no information on the transmission channel properties. In diversity coding methods, a data stream is coded using techpiques called space-time coding. The diversity coding exploits that signals of the various multiple antenna communication links are fading uncorrelated or independently. Diversity coding can be applied also when using spatial multiplexing.
In MIMO combining or multiplexing, it is particularly useful to incorporate information on the MIMO communications channel properties usually referred to as channel information. However, such channel information is not required when using diversity coding.
In communications it is since long well established practice to use a complex representation of signals, the magnitude and phase of which determines the magnitude of an in-phase component and a quadrature-phase component in a complex plane. The in-phase and quadrature-phase components represent a weighted sum of base functions. In this context orthogonal base functions over a symbol interval are preferred. Examples of such orthogonal base functions are sine and cosine waveforms.
It is convenient to represent multiple data streams using vector terminology and vector algebra. This is merely a simplified model of actual implementations to make understanding of the underlying principles easier and is not reducing the actual technology to mathematic theories. A concept well known from the vector algebra is the null space of an operator. The null space of an operator A is the set of all operands v which solve the equation Av=0. If the operator is a linear operator on a vector space, the null space is a linear subspace. The null space is then a vector space. If A is a matrix, the null space is a linear subspace of the space of all vectors. The dimension of this linear subspace is called the nullity of A. The rank-nullity theorem states that the rank of any matrix plus its nullity equals the number of columns of that matrix.
The null space of A can be used to find and express all solutions (the complete solution) of the equation Ax=b. If x1 solves this equation it is called a particular solution. The complete solution representing all possible solutions of the equation is equal to the particular solution added to any vectors from the null space.
FIG. 2 illustrates schematically example relaying comprising cooperative relaying for communications from a casting entity <<TX>> to a capturing entity <<RX>>. Communications are distributed along two routes 1 and 2. Route 1 comprises two wireless hops <<1a>>, <<1b>> and includes a relay station 1 <<RS1>>. Correspondingly, route 2 comprises two wireless hops <<2a>>, <<2b>> and includes a relay station 2 <<RS2>>.
U.S. Patent Application US20050014464 relates to wireless networks using relaying. Forwarding, at a relay station, of signals from a first link between a transmitter and the relay station is adapted as a response to estimated radio channel characteristics of at least the first link. The U.S. Patent Application includes embodiments with relay stations with multiple antennas in each relay station.
3rd Generation Partnership Project (3GPP): Technical Specification Group Radio Access Networks, Universal Terrestrial Radio Access (UTRA) repeater planning guidelines and system analysis (Release 6), 3GPP TR 25.956 V6.0.0, France, December 2004, describes planning guidelines and system scenarios for UTRA repeaters. In addition, it also contains simulations and analysis of the usage of repeaters in UMTS networks. Section 5.1.1. discusses antenna isolation. As a repeater amplifies a received signal, it can act as an oscillator under certain circumstances. The feedback path in this oscillating amplifier system is established through the two antennas of the repeater: coverage antenna and donor antenna. According to the 3GPP technical specification, the optimum in order to minimize the risk of oscillations is a combination of donor and coverage antennas that are mounted the way that there is a null in the antenna pattern in the direction pointing towards the other antenna. A null means minimum antenna gain in the specified direction.
As both antennas are usually mounted in opposite directions, it is useful to choose both donor and coverage antenna types that have a high front-to-back ratio.
Typical antennas that are used for repeater sites have a narrower aperture in the vertical antenna pattern. The vertical distance of the antenna influences the isolation of the antenna system. In a typical configuration, when both antennas are mounted on a pole, there is a null in the antenna pattern pointing vertically up and down from the antenna's feeding point. If there is a horizontal separation between the antennas, additional lobes in the vertical antenna pattern have to be taken into account.
Reflection and attenuation properties of all materials near the antenna can influence the antenna isolation of a repeater drastically.                Waves transmitted by antennas are reflected by surfaces, depending on the materials. If there is a reflection from a building towards a pole with the mounted antennas, this can decrease the antenna isolation by more than 10 dB.        The material of an antenna tower itself has also an effect on the isolation: If both antennas are mounted on a tower made of concrete, this improves the antenna isolation, as signals are attenuated and reflected by the material of the tower. A steel grid tower however might not increase antenna isolation particularly, as the distances between the single elements in the tower might be bigger than half a wavelength, which means that radiated power can pass the tower almost unattenuated. In this case, antenna isolation is more dependent on the antenna patterns.        Shielding grids mounted near the antennas also have an effect on the isolation. Generally, isolation can be improved by approximately 5 dB using a shielding grid. This depends on the shape of the shielding grid. Grids that are shaped according to the antenna outlook are better than simple ones.        
Section 5.1.4 considers repeater delay. The UTRA BS and UEs can handle a 20 μs time delay between two paths (c.f. TS 25.101 and TS 25.104). The repeater introduces a time delay of 5-6 μs. The signal paths introduced through the repeater will be longer than the direct path, both due to the extra traveling distance required for the signal (approximately 5 μs per 1.5 km) and due to the group delay in the repeater itself. For outdoor repeater coverage, where the areas can be substantial, it is a rule of thumb that the repeater site should be placed between the repeater service area and the donor base station.
Section 5.2 concerns coexistence of two or more uncoordinated operators. Consider a base station BSA and BSB of operators A and B, respectively, and user equipment UEA and UEB connected to operator A and B, respectively. In the case where a repeater is installed in the vicinity of a base station operating on the adjacent channel the required isolation between base station B and the repeater's donor port will depend on the up-link gain. Anyone can imagine that a signal transmission from UEB propagating towards BSB can take two routes to BSB—directly or via repeater A of operator A—both of which routes may comprise multi-paths. In the 3GPP technical specification, it is assumed that the path through the repeater will be badly distorted, since the delay spread in the adjacent carrier frequencies are bound to be substantial. As a consequence, the signal traveled through the repeater is considered to be interference only. However, this interference will have a fixed relation to the signal power from UEB arriving in BSB since it follows the power control applied from BSB on UEB. In the 3GPP technical specification, this phenomenon is denoted self-interference.
SsIR (Signal to self-Interference Ratio) is the relation between the power level of the distorted UEB signal from the repeater A path and the undistorted signal arriving directly from the UE, taken on the BSB receiver terminal. The effect of putting the SsIR to 0 dB is that a UEB with this SsIR occupies twice the air interface capacity from BSB as required from a UE with infinite SsIR utilizing the same service, resulting in a minimum requirement for the isolation between the repeater A donor port and the base station B receive port.
RA2/Private Business Systems Unit, Radiocommunications Agency, Wyndham House, 189 Marsh Wall, London, E14 9SX, ‘RA269,’ July 1999, published on the Internet, discusses the concept of on-frequency repeaters. An on-frequency repeater, OFR, is considered to be a device that receives an RF (radio frequency) signal and re-transmits or re-radiates it on the same frequency without any significant delay.
OFRs can re-transmit or re-radiate unwanted signals as well as wanted signals, and so cause interference. Additionally, because of the nature of their operation, OFRs are prone to instability. If they are not site engineered with a high degree of care, oscillation may occur and lead to serious interference to the licensee and other users of the radio spectrum.
U.S. Patent Application US20020045431 describes variable gain control in an on-frequency repeater. A narrowband detector is adapted to detect respective RF signals within each of a first and second wideband signal paths. Finally, the micro controller operates to control each of the AGC (Automatic Gain Control) blocks using the detected RF signals. A slaved variable gain amplifier of one wideband signal path is arranged to selectively amplify RF signals in the respective wideband signal path based on a signal power of RF signals in the other wideband path. The repeater comprises a directional donor unit and a subscriber coverage unit. The directions donor unit operates to establish and maintain a network link between the repeater and the base station. The transmit and receive performance of the directional donor unit enable maintenance of the network link with the base station even when the directional donor unit is located well beyond the conventional cell and/or network coverage area boundary. A feedback path within the uplink AGC closes the control loop of the AGC and limits system oscillation by automatically adjusting gain of the variable gain amplifier in the event of inadequate isolation between the directional donor antenna and the subscriber coverage antenna.
U.S. Patent Application US20050232194 also describes variable gain control in an on-frequency repeater. A narrow-band signal within a broadband RF signal is identified and isolated. The isolated narrowband signal is then processed to detect repeating features of the narrowband signal, thereby recognizing and identifying the signal type. System gain of the on-frequency repeater can be controlled based on the power level of the identified narrowband signal.
FIG. 3 illustrates schematically coverage areas <<S1>>, <<S2>>, <<S3>> and <<Coverage Area>> of three sectors of a base station <<BTS>> and a repeater station <<RS>>. The repeater station <<RS>> is equipped with an antenna <<Donor Antenna>> directed towards the base station <<BTS>>, and an antenna <<Coverage Antenna>> providing radio coverage in the area <<Coverage Area>> served by the repeater station.
The donor and coverage antennas are transmitter and receiver antennas, respectively, in uplink direction. In downlink direction the donor and coverage antennas are receiver and transmitter antennas, respectively.
Andrew Corporation: Bulletin BR-101111.1-EN (04/06), 2006, promotes a repeater for on-frequency operations. The repeater comprises automatic feedback interference cancellation, canceling at least partially from a composed input signal a feedback component of the input signal that is fed back to the repeater input from the repeater output signal. In the bulletin, the automatic interference cancellation equipment, ICE, is claimed to be capable of 35 dB enhancement of antenna isolation. Output to input isolation in the range of 95 dB (including ICE enhancement) is mentioned for a repeater providing a link gain of approximately 80 dB. An output to input isolation 15 dB greater than the gain provided is a common requirement for this kind of repeaters.
The repeater described in Bulletin BR-101111.1-EN is an example of a fairly traditional cancellation of signals interfering with desired signals at the input of a receiver. J. Chun, J. Lee, P. Choi, J. C. Yun, S. J. Lee, J. H. Lee, ‘Smart Antennas for the On-Air On-Frequency Repeater in the 3G Mobile Communication Applications,’ Proceedings of SPIE Vol. 4474, 2001, pp. 376-383, presents a beamforming algorithm that can be used for an on-air on-frequency repeater in an attempt to solve a well-known problem of LCMV (Linear Constrained Minimum Variance) beamformers suppressing the desired signal. The authors reveal an on-frequency repeater comprising a plurality of donor antenna elements for making a fixed highly directive beam on the donor side, meanwhile limiting the signal power from the serving side due to the coverage pattern of the donor antenna, unless there are reflecting objects introducing interfering signal components from the serving side in a direction of the donor antenna's radiation beam pattern.
International Patent Application WO2005062427, relates to high data rate communications, and more especially to line of sight, LOS, multiple input multiple output, MIMO, communications links and antenna configuration for LOS MIMO links, particularly radio links and optical wireless links.
Matti Latva-aho, ‘Advanced receivers for wideband CDMA systems,’ Department of Electrical Engineering, University of Oulu, Finland 1999, considers advanced receiver structures capable of suppressing multiple-access interference in code-division multiple-access (CDMA) systems operating in frequency-selective fading channels. Linear minimum mean squared error, LMMSE, receivers are derived and analyzed in frequency-selective fading channels. Different versions of the LMMSE receivers are concluded to be suitable for different data rates.
None of the cited documents above discloses a method and system of interference mitigation for a repeater station suppressing potentially interfering output signal components of the repeater station such that they do not reach the signal input of the repeater station.